Cell cycling is essential for normal cell growth and proliferation. A mitotic cell cycle consists of five major phases, G0, G1, S, G2, and M. G0 is the resting phase of a non-replicating cell. Cells enter the replicating cell cycle by leaving the G0 phase at the G1 phase. G1 is the first gap period between cell birth at the end of the M phase, and the initiation of new DNA synthesis at the beginning of the S phase. G2 is the second gap period in the replicating cell cycle during which cells possess twice their original numbers of chromosomes. During the M phase, chromosomes condense and segregate, and nuclear envelopes break down and reform. At the end of the replicating cell cycle, each parent cell divides into two replica daughter cells with identical numbers of chromosomes and essential organelles.
The mitotic cell cycle is controlled by phosphorylation of proteins essential in specific cell cycle events. A group of heteroproteins are known to be responsible for regulating DNA replication and the initiation of mitosis through their protein phosphorylation activity. These heteroproteins consist of a regulatory subunit, cyclin, and a catalytic subunit, cyclin-dependent kinase (CDK). In particular, cyclins contribute to the specificity of substrate protein binding by forming part of substrate protein-binding surface. Cyclins are phase-specific: as the cell enters the next phase of the cycle, cyclins specific to the previous phase are rapidly degraded. CDKs depend on their association with a cyclin for kinase activity. Although the levels of CDKs in mammalian cells rise and fall during the course of the cell cycle, the same CDKs remain throughout the full cell cycle and are not phase-specific. Together, the cyclin-CDK complexes control cell cycle events particular to DNA synthesis and mitosis through phosphorylating multiple proteins important in regulating the cell cycle events.
Protein kinases also function as checkpoint controls to ensure the proper completion and initiation of each phase in the cell cycle. These kinases arrest the cell cycle when a DNA mutation is detected. For example, p53 is a tumor suppressor which arrests cells with damaged DNA at the G1 checkpoint. Specifically, p53 functions as a transcription factor which stimulates the transcription of a number of genes encoding proteins such as cyclin-dependent kinase inhibitors (CDKIs). CDKI inhibits the function of CDK-cyclin complexes at the G1 phase, and thereby holds cells from progressing in the cell cycle until the damaged DNA is repaired and the p53/cyclin-dependent kinase inhibitor level falls. When the p53 checkpoint does not function properly, however, cells continue to divide in spite of damaged DNA, producing a body of transformed cells. Mutations of the p53 gene have been closely correlated with human metastatic cancers.
MP44 is a 44 kDa phosphoprotein associated with Xenopus mitosis. (Stukenberg, P. T. et al. (1997) Curr. Biol. 7: 338-348.) Antibody recognition and electrophoretic mobility shifts studies suggest that MP44 has three potential mitogen-activated protein kinase phosphorylation sites and a strong tendency for binding Cdc2, an essential mitotic cyclin-dependent protein kinase. These findings suggest that MP44 plays a potential role in transcriptional or translational regulation during mitosis. Another phosphoprotein, VASP, is a vasodilator-stimulated phosphoprotein identified in both human and mouse. (Zimmer, M. et al. (1996) Genomics 36: 227-233.) VASP is a potential regulator of translation or mRNA targeting.
The discovery of two new human cell-cycle phosphoproteins and the polynucleotides which encode them satisfies a need in the art by providing new compositions which are useful in the diagnosis, prevention, and treatment of inflammation and disorders associated with cell proliferation and apoptosis.